This invention relates to a hydraulic shock absorber and more particularly to an improved variable damping force hydraulic shock absorber.
Conventional hydraulic shock absorbers are well known in the art. Conventional hydraulic shock absorbers include a piston that reciprocates within a hollow cylinder. The piston is fixed to one end of a piston rod and the piston is disposed within the internal space of the cylinder to divide the cylinder into first and second chambers. The chambers are typically filled with a fluid such as oil to resist the motion of the piston within the cylinder. The reciprocating movement of the piston is resisted because the fluid must flow through a resistance mechanism when flowing from one chamber to the other chamber. Typically, the resistance mechanism comprises throttle plates or check valves which control the damping of the shock absorber. The movement of the fluid through the resistance mechanism dissipates the input energy to the shock absorber by displacing the fluid through the resistance mechanism. The velocity of the reciprocating piston, which determines the amount of energy dissipated, is controlled by the amount of resistance to the fluid flow.
It has been recognized that the ability to change the damping characteristics of the hydraulic shock absorber is advantageous and desirable. Variable damping force hydraulic shock absorbers often include an additional fluid communicating path that bypasses the resistance mechanism of the shock absorber and connects the first and second fluid chambers of the shock absorber. The communicating path is often located within the piston rod and establishes an alternative fluid path of less resistance between the first and second chambers of the shock absorber. This fluid communication path decreases the damping of the shock absorber because the fluid flowing through the communicating path is not subject to the resistance mechanism of the shock absorber.
It is known to adjust the damping of the shock absorber by controlling the amount of fluid flowing through the communicating path. For example, Japanese unexamined patent publication Hei4-78338 discloses a conventional hydraulic shock absorber with a piston that reciprocates in a cylinder filled with fluid and a fluid communicating path located in the piston rod that connects the first and second fluid chambers. The fluid communicating path allows the fluid to bypass the resistance mechanism of the piston and this decreases the damping of the piston. The Japanese patent application also discloses mounting a solenoid driven control valve on the piston rod to vary the size of the opening to the fluid communicating path bored through the piston rod. The solenoid driven control valve includes a solenoid and a plunger, which are axially aligned with the opening to the communicating path. The movement of the plunger is controlled by the solenoid and the solenoid moves the plunger into and out of the opening to the communicating path to control the fluid flowing through the passage.
In further detail, the Japanese patent application discloses that when the solenoid is energized, the solenoid removes the plunger from the opening to the communicating path. This decreases the damping force of the shock absorber because the fluid flows through the communicating path and the fluid bypasses the resistance mechanism of the shock absorber. Alternatively, when the solenoid is deenergized, a spring pushes the plunger into the opening of the communicating path and this forces the fluid to flow through the resistance mechanism of the piston. Accordingly, the damping force of the shock absorber is comparatively greater. Thus, a variable damping force hydraulic shock absorber which allows larger and smaller damping forces to be obtained by energizing and deenergizing the solenoid driven control valve is known.
It is also known to adjust the damping force of the shock absorber by increasing or decreasing the amount of electrical current flowing to the solenoid. This allows the solenoid to move the plunger in small steps such that the size of the opening to the communicating path is gradually changed. Consequently, the damping force of the shock absorber is variously adjustable because the rate of fluid flow through the communicating path can be changed in small increments.
This proposed arrangement, however, can be improved upon because the damping characteristics of the adjustable shock absorbers can vary from unit to unit, especially when the shock absorbers are mass produced, The amount of damping force provided by variable damping force hydraulic shock absorbers varies because of the inherent variability and inconsistency of manufacturing process. The amount of damping may vary, for example, because of the different properties of the electric components, such as the solenoids, and the different shapes and sizes of the component parts used to construct the shock absorber.
It is, therefore, a principal object of this invention to provide an improved variable damping force hydraulic shock absorber.
It is desirable when producing variable damping force hydraulic shock absorbers to be able to adjust the amount of damping force provided by each shock absorber such that a uniform amount of damping is provided by each shock absorber. A consistent amount of damping force permits the shock absorbers to be interchangeable and a specific relationship between the magnitude of the electrical current supplied to the shock absorber and the amount of damping force provided by the shock absorber to be established. For any given amount of electrical current supplied to the adjustable shock absorber, the shock absorber should respond in a predictable manner to provide a specified amount of damping.
It has been proposed to supply a different amount of electrical current to each shock absorber to obtain a uniform amount of damping. This would permit each shock absorber to have a constant initial amount of damping and any additional electrical current supplied to the shock absorber would change the damping characteristics of the shock absorber. This proposal, however, requires a complex arrangement to supply various amounts of electrical current to the different shock absorbers to obtain a uniform amount of damping. Alternatively, the shock absorber must be disassembled and the internal components exchanged or adjusted to obtain the desired uniform amount of damping.
It is, therefore, a further object of this invention to provide a variable damping force hydraulic shock absorber that is externally adjustable to provide uniform damping.